a) Field of the Invention
The present invention relates to a solid state image pickup device, and more particularly to a solid state image pickup device of high resolution having a number of photoelectric conversion elements, and a method of driving such a solid state image pickup device.
b) Description of the Related Art
Extensive developments of solid state image pickup devices have been made particularly for video cameras. High resolution is one of such developments. Solid state image pickup devices having 2 to 4 millions pixels have been developed for use with high definition televisions (HD-TV), mechanical measurements, astronomical observations, and the like.
However, solid state image pickup devices presently used in practice for commercial color cameras typically of 525 vertical scan lines are only those having four hundred thousands pixels or less.
Solid state image pickup devices having more pixels are desired when considering applications to still video cameras, image input apparatuses, electronic overhead projectors (OHP) and the like which would require vertical scan lines more than 525.
The present applicants have studied an optimum number of pixels from theoretical calculations and image simulation, and reached a conclusion that the number of pixels about eight hundred thousand is sufficient for such applications. Based upon this conclusion, various proposals have been made for CCD color sensors having about eight hundred thousand pixels.
For an image having an aspect ratio of about 3:4, it has been found that an optimum layout of eight hundred thousand pixels is about 1000 pixels in the vertical direction and about 800 pixels in the horizontal direction.
Specifically, about 1000 photodiodes in the vertical direction and about 800 photodiodes in the horizontal direction are disposed in a matrix form. Vertical CCD (VCCD) columns for transferring electric charges in the vertical direction are provided in juxtaposition with photodiode columns. A horizontal CCD (HCCD) row for transferring electric charges in the horizontal direction is coupled to the output terminals of the VCCD columns.
In order to read electric charges from about 1000 pixels disposed in the vertical direction, VCCD is used which has two transfer cells per one row for example.
A method of reading image information in a short time period has been proposed, whereby photodiodes in the vertical direction are classified into four types, and electric charges from two types of photodiodes are read at a time. With this method, information of all pixels can be read in 2 V periods where V is a vertical scan period. In this case, because the requirement to be met when transferring electric charges the horizontal direction, two HCCD rows are provided to transfer two types of electric charges at a time.
In the system using two HCCD rows, the intensity of image signals changes depending on which HCCD row was used for charge transfer. This intensity level difference of image signals appears as a flicker phenomenon particularly for a three chip color image pickup device.
In order t,o avoid such a phenomenon, the present applicants have proposed a method of reading all image information in 4 V periods where V is a vertical scan period. With this method, it is possible to reproduce a still image with a high precision.
During a monitor mode prior to taking a still image (including a movie mode for taking moving images), an image reproduced by using the NTSC television method to reproduce it in a short time.
FIGS. 2A to 2D illustrate a high resolution solid state image pickup device according to the conventional technique.
FIG. 2A schematically shows the structure of a image pickup device. A number of photodiodes 3 are disposed in a matrix form. For example, about 800 photodiodes are provided for each row of about 1000 rows. Photodiodes 3 of each row are classified into four types A1, B1, A2, and B2 in the descending order in each column.
VCCD 4 is provided in juxtaposition with each photodiode 3 column. VCCD 4 has two transfer cells per one row, eight cells of four rows forming one unit. Insulated electrodes are formed on transfer cells along the semiconductor transfer channel. Transfer cells of one unit are applied with control signals V1, V2, V3, V4, V5, V2, V6, and V4 in the descending order in each column.
Electric charges accumulated in each photodiode are moved to a corresponding cell of an adjacent VCCD 4 by applying a high voltage to the cell. The electric charges in VCCD 4 are transferred downward in the vertical direction by sequentially and selectively applying predetermined voltages to the electrodes of VCCD 4.
HCCD 101 is connected in common to the lower ends of all VCCDs 4. Electric charges transferred downward in each VCCD 4 are transferred to HCCD 101 in response to control signals.
Shift gates SG 103 capable of selectively transferring electric charges are provided in juxtaposition with HCCD 101. Another HCCD 102 is provided under the shift gates SG 103.
The shift gates SG 103 and HCCD 102 under the shift gates SG 103 are omitted in an image pickup device which reads electric charges stored in all the photodiodes 3 in 4 V periods.
FIG. 2B illustrates the mode of reading electric charges stored in all the photodiodes 3 in 2 V periods. Electric charges of photodiodes A1 and A2 are read during the first V period, and those of photodiodes B1 and B2 are read during the next V period. In this manner, electric charges of all the photodiodes 3 can be read in 2 V periods.
In this mode, electric charges stored in photodiodes A2 and B2 are attenuated by a transfer loss at the shift gates SG 103, whereas electric charges stored in photodiodes A1 and B1 are not subject to such attenuation. In addition, a difference between characteristics of HCCD 101 and HCCD 102 and a difference between amplifier characteristics of output amplifiers FDA 1 and FDA 2 are superposed to the attenuation.
FIG. 2C illustrates the mode of reading electric charges of all photodiodes in 4 V periods. Electric charges of photodiodes A1 are read during the first V period, those of photodiodes A2 are read during the second V period, those of photodiodes B1 are read during the third V period, and those of photodiodes B2 are read during the fourth V period.
In this charge reading mode, all electric charges are read via only HCCD 101 and FDA1, and so it is easy to make uniform the charge read characteristic. In the case of an instant exposure, a strong light is radiated to a solid state image pickup device. Therefore, smear electric charges are generated in CCD of the image pickup device. If signal charges are picked up without considering smear electric charges, signals picked up at the first field contain smear electric charges which can be neglected at other fields.
FIG. 2D illustrates the mode of reading electric charges in 4 V periods under an instant exposure. After an instant exposure by an operation of an electronic flash lamp, mechanical shutter or the like, smear electric charges in CCD are swept out during the next V period. During the following 4 V periods, signal charges are read from photodiodes in the similar manner described with FIG. 2C.
In the case of an instant exposure, after smear electric charges are swept out, signal charges are read in 4 V periods as described above. Therefore, the timings of picking up signal charges differ between the continuous exposure and instant exposure. Information discriminating between the continuous exposure and instant exposure is transmitted via one cable connecting a camera head and a camera controller, respectively of a head separate type solid state image pickup device.
In the monitor mode, prior to taking a still image, for determining a framing or in the movie mode for taking moving images, a simple and speedy image pickup method is desired even for an image pickup device of an HD-TV system.
FIGS. 3A to 3D illustrate the monitor mode (inclusive of the movie mode) using the 4 V read method. In the monitor mode, the NTSC method is desirable in order to reproduce an image easily and speedily.
FIG. 3A illustrates monitoring by the NTSC method using a monitor of 500 vertical scan lines. Image signals supplied from photodiodes A1 and A2 during the first 2 V periods form an image for the photodiodes A1 and A2. During the next 2 V periods, electric charges from photodiodes B1 and B2 are supplied to form a corresponding image.
The image 105a for the photodiodes A1 and A2 differs in position by one vertical scan line from the image 105b for the photodiodes B1 and B2. Therefore, with the 4 V read method which is used in taking a still image, a vertical jitter is produced in an image on a monitor of 500 vertical scan lines.
FIG. 3B illustrates monitoring an image using a monitor of 1000 vertical scan lines. Signals obtained by the 4 V read method are supplied to the monitor of 1000 vertical scan lines to form one frame image 106 corresponding to the electric charges of photodiodes A1, A2, B1, and B2.
It takes 4 V periods to read image signals of one frame. In reproducing one frame image by the HD-TV method, all image signals are stored temporarily to process them, thereby adding another process time. Therefore, use of a monitor of 1000 vertical scan lines poses a problem of a low motion resolution and a long processing time.
It is desirable to use a monitor of 500 vertical scan lines and the NTSC method in order to reproduce an image easily anti speedily. The methods as illustrated in FIGS. 3C and 3D have been proposed for the NTSC image reproduction by using an :image pickup device of 1000 vertical scan lines.
According to the method illustrated in FIG. 3C, accumulated electric charges are read in the similar manner used when taking a still image. However, in reproducing an image, electric charges of only photodiodes A1 and A2 are used. Namely, outputs of CCD are supplied sequentially in the order of A1, A2, B1, and B2, and these read-out image signals are written in a memory.
All image signals written in the memory are not used for monitor signals, but signals from the photodiodes A1 and A2 only are repetitively read. Specifically, when the image signal read from the photodiode A1 is written in the memory, the previously stored image information A2 is read from the memory, and when the image signal read from the photodiode A2 is written in the memory, the previously stored image information A1 is read from the memory.
During the period to be otherwise used for reading image signals B1 and B2, the image signals A2 and A1 are read from the memory. In this manner, a 500 line monitor image can be reproduced without a vertical jitter.
The above description, while an image signal for the photodiode A1 is written in the memory, signal A2 is read, and while the image signal A2 is written, signal. A1 is read. Alternatively, image signals read from the photodiodes A1 and A2 may be directly supplied to the monitor, and while image signals for the photodiodes B1 and B2 are read, the previously stored image signals A1 and A2 may be read from the memory.
This method can prevent a vertical jitter. It is however difficult to improve the motion resolution.
FIG. 3D illustrates the pixel mixed type read method. For the first field corresponding to the first V period, image signals are read from the photodiodes A1 and B1 and mixed together to generate monitor signals. For the second field corresponding to the next V period, image information is read from the photodiodes A2 and B2 and mixed together to generate monitor signals. In this manner, all pixel information is read in 2 V periods.
With this method, a vertical jitter does not occur and a monitor image of high motion resolution can be obtained. All accumulated electric charges are read in 2 V periods, so that the charge accumulation period is 2 V periods. Since the charge amount is doubled by pixel mixture, the obtained charge amount is not the same as the case where image information is read From each pixel in 4 V periods (accumulation period of 4 V) under the continuous exposure.
In reading image information through pixel mixture, electric charges accumulated in two photodiodes are mixed and transferred by HCCD. Therefore, the drive voltage of HCCD is required to have a voltage sufficient for driving two-fold saturated electric charges.
Namely, if strong light is applied, each photodiode accumulates electric charges whose amount is approximately equal to the saturated electric charges. IF these saturated electric charges are mixed together, it is necessary for HCCD to transfer electric charges two times as much as the saturated electric charges. If HCCD is driven by a drive voltage capable of transferring the saturated electric charges one photodiode, deficient charge transfer may occur to leave residue charge in the cell.
It is necessary for an HCCD drive voltage swing to be boosted two-fold for example. The doubled swing voltage and high speed charge transfer may cause a not-negligible power loss because of a not-negligible capacitance C of the CCD structure.
As described above, a 500 line monitor with a conventional 1000 line image pickup device using the NTSC method may cause a vertical jitter, a lower motion resolution, a longer process time, a power loss, and the like.
Furthermore, in taking a still image, the period required for reading images changes between 4 V periods and 5 V periods depending upon whether an instant exposure or a continuous exposure is used. In order to discriminate the exposure type, an identification signal should be supplied. For a camera head separate type solid state image pickup device, this identification signal requires one transmission cable.